The use of supercritical fluids in industrial processes has been growing at an ever-quickening pace. Replacing traditional, often hazardous and flammable, organic solvents with supercritical fluids has been a prime area of research. Carbon dioxide is a popular choice due to the fact that it is nontoxic, nonflammable, and inexpensive. An attractive feature of a supercritical fluid is that its density can be varied simply by changing the pressure or temperature. Therefore, all density-dependent properties, such as dielectric constant and solubility parameter, can be manipulated in this manner. These key features of supercritical fluids make them ideal candidates for use in extraction and chromatography applications.
In the chemical and pharmaceutical industries, the demand for purified compounds is increasing steadily. It has become highly desirable to obtain components of the highest available purity in the largest quantities. In many instances, high performance liquid chromatography (HPLC) has been the analytical method of choice for these types of separations. HPLC can be analytical or preparative in nature, the component levels varying depending on the application. In the case of preparative HPLC, a collection means is also employed for the sample fractions. However, a drawback to the use of HPLC is the fact that in many instances long elution times, as well as large of amounts of solvents are required for the process.
Supercritical fluid chromatography (SFC) has been introduced in the past decade as an alternative to HPLC. The technique employs a supercritical fluid, typically carbon dioxide, as the mobile phase. In many instances, an organic solvent is also present as a modifier in order to adjust the polarity of the mobile phase. Because supercritical fluids are known for their high diffusivities, this results in enhanced speeds and resolving power when compared to HPLC. The difference can be as much as an order of magnitude in some applications. Additionally, SFC systems can reequilibrate faster than HPLC systems and therefore can be ready to process other samples in a shorter time frame. Many of the advantages of SFC over HPLC are applicable to both analytical and preparative methods. However, much like HPLC, SFC also needs a means to collect the sample fractions, preferably multiple fractions, in an efficient, cost-effective manner.
Collection means for SFC have been explored in past research. For example, U.S. Pat. No. 6,413,428 (Berger et al.) and EP 117057 (Berger et al.) disclose a sample collection process for preparative SFC using a collection chamber consisting of test tubes. The system is automated in an embodiment. Sample collection is also discussed in U.S. Pat. No. 5,601,707 (Clay et al.), U.S. Pat. No. 6,086,767 (Walters et al.), U.S. Pat. No. 6,309,541 (Maiefski et al.), and U.S. Pat. No. 5,614,089 (Allington et al). While there are several mechanisms for analyte collection in SFC, there still exists a need which can collect all fractions more efficiently and concentrate the samples for further analysis. Implementing such a method would make the overall process of SFC much more cost-effective.
U.S. Pat. No. 5,205,987 (Ashraf-Khorassani et al.) disclosed a collection mechanism for off-line supercritical fluid extraction in which the analyte is gathered in a collection trap after being extracted. The collection means contained beads to trap the analyte and carbon dioxide was used to cool the trap. The analyte was then desorbed from the trap using an appropriate solvent. A vial was placed downstream from the trap to collect the analyte dissolved in the desorbing solvent. However, this patent does not address the issue of concentrating the samples. The object of the present invention is to apply the engineering fundamentals of this patent to a collection system for chromatography in which the samples are also collected at a higher concentration.